Carbon fiber paper for solid polymer fuel cells

ABSTRACT

The present invention provides carbon fibre paper for use as current collectors for a polymer electrolyte fuel cell. 
     Specifically, it relates to a carbon fibre paper in a polymer electrolyte fuel cell in a state where short carbon fibres are bound with a polymer material and, taking the thickness as X mm, the thickness at the time of 2.9 MPa applied pressure as Y mm and the length of the short carbon fibre as W mm, at least 95% of the short carbon fibres excluding those of length (Y+0.1) mm or less satisfy the relation W≧5X. Taking the average length as Z mm, it is also preferred that the relation Z≧5X also be satisfied, and the short carbon fibre diameter D (μm), tensile strength σ (MPa) and tensile modulus (E) satisfy the following relation: 
     
       
         σ/( E×D )≧0.5×10 −3 .

TECHNICAL FIELD

The present invention relates to carbon fibre paper which is used as thecurrent collector of a polymer electrolyte fuel cell.

TECHNICAL BACKGROUND

In addition to a current collecting function the current collectors of apolymer electrolyte fuel cell need to allow diffusion/permeation of thesubstances which participate in the electrode reactions. Furthermore,the material from which the current collectors are composed needs topossess electroconductivity and the strength to withstand gasdiffusion/permeation and handling, and also the strength to withstandcompression at the time of electrode production and during cellassembly, etc. When compared to the characteristics demanded of tilecurrent collectors for a phosphoric acid fuel cell, in the case of thecurrent collector of a polymer electrolyte fuel cell the strength of thepolymer electrolyte membrane is high so the strength of the currentcollectors need only be enough to withstand handling, Furthermore, thecorrosion resistance need only be low, so the polymer material selectionrange is broad. However on the other hand, since the strength andresistance of the polymer electrolyte membrane is high, its thickness isreduced, so it is necessary that there be no projecting regions in thecurrent collectors which could cause a short circuit through the polymerelectrolyte membrane. Additionally, since the polymer electrolytemembrane the catalyst layers and current collectors are usually coupledtogether by application of pressure, it is necessary that the currentcollectors be undamaged, not merely by the applied pressure at the timeof cell assembly but also by the pressure at the time of the integralcoupling, and it is necessary that a short circuit does not occurthrough the polymer electrode membrane.

As the material used for the current collectors of such a polymerelectrolyte fuel cell, there is known a porous carbon sheet formed bybinding together short carbon fibre with carbon, as described inJapanese Unexamined Patent Publication (Kokai) Nos 6-20710, 7-326362 and7-220735. However, since such porous carbon sheet is produced by firstlypreparing an aggregate of short fibre comprising carbon fibre or carbonfibre precursor fibre, then impregnating or mixing with resin andfiring, the production costs are high. Again, in the case where thedensity is low, there is also the problem that the binding carbon isreadily damaged by the pressure applied at the time of electrodeproduction or cell assembly.

The use of a paper-form short carbon fibre aggregate as a currentcollector is proposed in Japanese Unexamined Patent Publication (Kokai)No. 7-105957 as a method for resolving the problem of production costs.With such a current collector since binding with carbon is not carriedout, it is necessary to apply a thickness direction pressure not just atthe time of the integral coupling of the polymer electrolyte, catalystlayers and current collectors, but also when used as a cell in order tolower the electrical resistance in the thickness direction. However, inthese inventions, no consideration is given to the lowering of theresistance or to preventing damage to the current collector at the timeof pressure application. Moreover, since the short carbon fibres arerandomly arranged when pressure is applied at the time of the electrodeproduction or cell assembly, short carbon fibres facing in the thicknessdirection readily pierce the polymer electrolyte membrane, bringingabout a short circuit with the facing electrode, and breakage of theshort carbon fibre also readily occurs. Again, in Japanese UnexaminedPatent Publication (Kokai) No. 8-7897, there is described the adhesionof the short carbon fibre in a state of entanglement with carbonparticles contained in the diffusion layer at the surface on thediffusion layer side of a coupled body comprising the electrolytemembrane forming the electrodes, the catalyst reaction layers anddiffusion layers, but since the short carbon fibre is fixed byentanglement with carbon particles in the diffusion layer, the shortcarbon fibres emerging at the surface are all at an angle in the termsof the plane of the coupled body, and at the time of pressureapplication when assembling the cell the short carbon fibres readilypierce the polymer electrolyte membrane bringing about a short circuitwith the facing electrode, and breakage of the short carbon fibre alsoreadily occurs. Moreover, since the layer of short carbon fibre is thin,there is low gas diffusion/permeation in the planar direction of thelayer, and it is necessary to provide a diffusion layer separately.

The present invention has been made in view of the aforementionedproblems of the prior art and has as its objective to provide a carbonfibre paper for use as the current collectors in polymer electrolytefuel cells where there is little concern about a short circuit with thefacing electrode occurring, where there is little fear of damage bypressure application, where the resistance value is comparatively lowand which is also cheap.

DISCLOSURE OF THE INVENTION

Specifically, the present invention is characterized in that it is acarbon fibre paper used in a polymer electrolyte fuel cell in a state inwhich short carbon fibres are bound with a polymer material and takingthe thickness as X mm and the thickness when 2.9 MPa pressure is appliedas Y mm, at least 95% of the short carbon fibres excluding those oflength (Y+0.1) mm or less satisfy the relation W≧5X.

Taking the length of the short carbon fibre as W mm and the averagelength of the short carbon fibre excluding fibre of length (Y+0.1) mm orless as Z mm, the aforesaid carbon fibre paper preferably satisfies therelation Z≧5X.

Furthermore, it is preferred that the short carbon fibre besubstantially randomly oriented within a two dimensional plane.

Moreover, preferably the relation between the short carbon fibrediameter D (μm), the tensile strength a (MPa) and the tensile modulus E(MPa) satisfies the following relation.

σ/(E×D)≧0.5×10⁻³

Additionally, it is preferred that the average length of the shortcarbon fibre be at least 4.5 mm and at least seven times the thicknessof the carbon fibre paper, and that the following relation be satisfied.

σ/(E×D)≧1.1×10⁻³

Furthermore, it is preferred that the short carbon fibre be short fibreof polyacrylonitrile-based carbon fibre, and it is preferred that thediameter of this short carbon fibre be no more than 20 μm and that thevolume resistivity in the short carbon fibre lengthwise direction be nomore than 200 μΩ.m.

Again it is preferred that the reduction in weight be no more than 3%when a uniform pressure of 2.9 MPa is applied in the thickness directionfor 2 minutes and then the pressure removed.

Moreover, it is preferred that the resistance be no more than 50 μΩ.cm²when a uniform pressure of 2.9 MPa is applied.

It is also preferred that the thickness is 0.02 to 2.0 mm and that thedensity lies in the range 0.3 to 0.8 g/cm³ when a uniform pressure of2.9 MPa is applied in the thickness direction, and it is preferred thatthe weight per unit area be in the range 10 to 100 g/m².

Additionally, it is preferred that the polymer material content lieswithin the range 2-30 wt % and it is preferred that fine carbonparticles also be included.

With regard to the current collector employing the carbon fibre paper ofthe present invention, individual units are constructed by arranging thecurrent collector and catalyst layer in the form of layers, and thepolymer electrolyte fuel cell is constructed from a stack containing aplurality of such units, and a moving body such as a motor vehicle orthe like may be driven by means of this polymer electrolyte fuel cell.

A current collector employing the carbon fibre paper of the presentinvention is produced, for example, by a method containing a stage inwhich, prior to the formation of the catalyst layer on the currentcollector, the carbon fibre paper is simultaneously heated and pressureapplied perpendicular to the plane of the carbon fibre paper, andpreferably, prior to the formation of the catalyst layer on the currentcollector, the carbon fibre paper is brought into contact with a liquidand the pressure applied in a state with the carbon fibre paper soakedwith liquid.

BRIEF EXPLANATION OF THE DRAWING

FIG. 1 is a side view of a polymer electrolyte type fuel cell relatingto an embodiment of the present invention.

Explanation of the numerical codes:

1: current collector

2: catalyst layer

3: polymer electrolyte membrane

4: separator

5: groove

OPTIMUM FORM FOR PRACTISING THE INVENTION

The polymer electrolyte fuel cell in the present invention is a fuelcell which uses a polymer electrolyte membrane as the electrolyte, andan example thereof is shown in FIG. 1. FIG. 1 illustrates a single cellcomprising respectively sheet-shaped current collectors 1, catalystlayers a and polymer electrolyte membrane 3, and the fuel cell isproduced by stacking a plurality of such single cells via groovedseparators 4. The catalyst layers are, for example, layers comprisingcarbon powder on which fine particles of a platinum type catalyst havebeen supported, bound together with a resin, and the thickness is about0.02 to 0.2 mm. The catalyst layers may be impregnated or mixed withpolymer electrolyte. Again, rather than being produced as sheet shapeson their own, it is better that the catalyst layers be formed on thepolymer electrolyte membrane or on the current collectors. The polymerelectrolyte membrane is for example a fluoropolymer type cation exchangeresin, of thickness about 0.05 to 0.15 mm. The interface between thepolymer electrolyte membrane and catalyst layer, or between the catalystlayer and current collector, may be produced by superposition or theremay be intermingling and the interface need not necessarily be clearlydelineated in the way indicated in FIG. 1. The separator is formed of anelectroconductive gas-impermeable material such as a carbon plate anelectroconductive plastic plate or the like and it has grooves 5 formedon both faces to produce channels for the fuel, air or water which isthe electrode reaction product. In FIG. 1, at the left side face of theseparator there is formed a groove in the lengthwise direction. Pressureis applied to the stacked plurality of single cells, in the stackingdirection, at the time of operation of the fuel cell. The pressure uspreferably from 0.5 to 10 MPa.

By binding the short carbon fibre with a polymer material, it ispossible to enhance the strength and handling properties of the carbonfibre paper and it is also possible to prevent shedding of short carbonfibre and to prevent the fibre being directed in the carbon fibre paperthickness direction. Again, by binding with a polymer material, there isenhanced strength in terms of compression or stretch.

As methods for the application of the polymer material, there are themethod of mixing a fibre-form, particle-form or liquid-form polymermaterial at the time of the production of the paper-shaped aggregate ofshort carbon fibres, and the method of applying a fibre-form,particle-form or liquid-form polymer material following the productionof the paper-shaped aggregate of short carbon fibres. The concept of‘liquid-form’ here includes materials such as emulsions, dispersionslatexes and the like, were fine particles of a polymer material aredispersed in a liquid and which can essentially be handled as a liquid.In order to strengthen the binding of the short carbon fibre or in orderto lower the electrical resistance of the carbon fibre paper, and henceof the current collector, it is preferred that the polymer material hasa fibre-form, or be an emulsion, dispersion or latex. In the case therea fibre-form polymer material is employed, it is preferred that filamentfibre be used to lower the amount employed.

The polymer material for binding the short carbon fibre is preferably apolymer material with carbon or silicon in the main chain, and there canbe used thermoplastic resins such as polyvinyl alcohol (PVA), polyvinylacetate, polyethylene terephthalate (PET), polypropylene (PP),polyethylene, polystyrene, polyvinyl chloride, polyvinylidene chloride,acrylic resins, polyurethanes and the like, thermosetting resins such asphenolic resins, epoxy resins, melamine resins, urea resins, alkydresins, unsaturated polyester resins, acrylic resins, polyurethanes andthe like, and also thermoplastic elastomers, butadiene/styrenecopolymers (SBR), butadiene/acrylonitrile copolymers (NBR) and othersuch elastomers, cellulose, pulp or the like. By using a fluoropolymeror other such waterproof resin, the carbon fibre paper may be given awaterproofing treatment at the same time as the binding of the shortcarbon fibre.

In order that damage does not readily occur at the time of theapplication of pressure to the current collector, it is better thatthere be used a soft polymer material to bind the short carbon fibreand, in the case where there is used a fibre-form or particle-formpolymer material, a thermoplastic resin, elastomer, rubber, cellulose orpulp is preferred. In the case where a liquid-form polymer material isemployed, a thermoplastic resin, elastomer, rubber, or a thermosettingresin modified by means of such as soft material, is preferred, with athermoplastic resin, elastomer or rubber being further preferred.

The polymer material preferably has an elastic modulus in compression at23° C. of 4,000 MPa or less, more preferably 2,000 MPa or less, andstill more preferably 1.000 MPa or less. A polymer material with a lowelastic modulus in compression mitigates stresses applied to the bindingregions and the binding does not readily fail. Again, stresses appliedto the short carbon fibres are mitigated and the short carbon fibres donot readily break.

Moreover, in the case where the polymer material is a crystallinepolymer material, its glass transition point (Tg) is preferably no morethan 100° C., more preferably no more than 50° C. and still morepreferably no more than 0° C. Above the glass transition point, thepolymer material does not crystallize so it is comparatively soft, andthe greater the temperature difference in respect of the glasstransition point the softer it is. The glass transition point Tg of acopolymer is determined by the following formula.

1/Tg=W ₁ /Tg ₁ +W ₂ /Tg ₂ −→W _(n) /Tg _(n)

Tg: glass transition point (K) of the copolymer

W_(n): weight fraction of monomer n

Tg_(n): glass transition point (K) of polymer formed by polymerizationof monomer n

In the case where water is used at the time of the processing of theundermentioned carbon fibre paper to produce the current collector or atthe time of the coupling thereof, in order to prevent the polymermaterial which binds the short carbon fibre from dissolving in the waterand the binding undergoing failure, it is preferred that awater-insoluble polymer material be used. Examples of water-insolublepolymer materials are polyvinyl acetate. PET, PP, polyethylene,polyvinylidene chloride, epoxy resin, unsaturated polyesters, SBR, NBRand the like. As a water-soluble polymer material, there can be used PVAbut, in such circumstances, it is preferred that it be mixed with someother polymer material, or a PVA copolymer be used, or that PVA with ahigh degree of saponification be employed. The degree of saponificationis preferably at least 85 mol %, and more preferably at least 95 mol %.

In the polymer electrolyte fuel cell, at the cathode (air electrode,oxygen electrode), there is generated water as the product of theelectrode reaction and also water which has passed through theelectrolyte. Furthermore, at the anode (fuel electrode), the fuel issupplied after moistening in order to prevent drying of the polymerelectrolyte membrane. Since dewing or build-up of such water, or theswelling of the polymer material by the water, inhibits the supply ofthe electrode reactants, the water absorption of the polymer materialshould be low. Preferably it is no more than 20% and more preferably nomore than 7%. Specifically, there is used PET, PP, polyethylene,polyvinylidene chloride, polystyrene, epoxy resin, unsaturatedpolyester, melamine resin, SBR, NSR or the like.

In order to prevent a lowering of the catalyst activity or a lowering ofthe electroconductivity of the polymer electrolyte membrane, thereshould be little impurity present In the polymer material. The weightratio of metal elements other than alkali metals (Li, Na, K, Rb, Cs,Fr), alkaline earth metals (Be, Mg, Ca, Sr, Ba, Ra), boron (B) andsilicon (Si) in the carbon fibre paper is preferably no more than 300ppm, more preferably no more than 100 ppm and still more preferably nomore than 50 ppm. Furthermore, the weight ratio of metal elements otherthan boron (B) and silicon (Si) is preferably no more than 1,000 ppm.more preferably no more than 700 ppm and still more preferably no morethan 500 ppm. As specific examples of polymer materials with littleimpurity, there are PET, PP, polyethylene, polystyrene and the like.Again, in the case where an elastomer is mixed, it is preferable thatthere not be used a material which contains sulphur as a vulcanizingagent.

The carbon fibre paper may be employed as it is as the currentcollector, and it may also be used after post treatment. Examples of thepost treatment are a waterproofing treatment in order to prevent alowering of gas diffusion/permeation due to the retention of water, apartial waterproofing or a hydrophilic treatment to form a waterdischarge channel, or the addition of carbon to bring about a loweringof resistance.

Taking the length of the short carbon fibres as W mm, it is necessarythat at least 95% of the short carbon fibres excluding those of length(Y+0.1) mm or less satisfy the relation W≧5X. More preferably this willbe at least 98%, and still more preferably at least 99%. Again, W≧7X ismore preferred and W≧12X is still further preferred. Where the value isless than 95%, then numerous short carbon fibres are disposed in thethickness direction of the carbon fibre paper, so that breakage of theshort carbon fibres or short circuits due to penetration of the solidelectrolyte membrane by the short carbon fibres occurs more readily.Here, the percentage of short carbon fibres is the numerical percentage.

The thickness X mm of the carbon fibre paper is measured based on JISP8118. The pressure at the time of measurement is made 13 kPa. Thethickness Y mm at the time of 2.9 MPa pressure application is determinedfollowing application of a uniform pressure of 2.9 Mpa, from thedifference in spacing of top/bottom pressure plates when the currentcollector is or is not interposed. In the measurement of the pressureplate spacing, the spacing between the pressure plates is determined bymeans of a micro-displacement detector at two edges containing thecentre point of the pressure plates, and the spacing calculated as theaverage of the spacings for the two edges. In order to apply a uniformpressure, one of the pressure plates is made to swivel so that the angleof the pressing faces of the top/bottom pressure plates is changeable.Short carbon fibre of length no more than (Y+0.1) mm may be arranged inany direction thee-dimensionally in the carbon fibre paper, butpenetration of the polymer electrolyte membrane or breakage does notoccur. From amongst such fibres, the short carbon fibres oriented in thecarbon fibre paper thickness direction have the effect of conferringunevenness at the catalyst layer or solid electrolyte membrane when thecarbon fibre paper is used as the current collector of a polymerelectrolyte fuel cell and so the electrode reaction area is increased,or such fibres have the effect of lowering the current collectorthickness direction resistance, so it is actually preferred that therebe included short carbon fibre of length no more than (Y+0.1) mm.

With regard to the length of the short carbon fibre, taking thethickness of the carbon fibre paper as X mm, the thickness at the timeof 2.9 MPa pressure application as Y mm, and the average length of theshort zarbon fibre excluding fibre of length no more than (Y+0.1) mm asZ, the relationship Z≧5X is preferably satisfied, more preferably Z≧7Xand still more preferably Z≧12X. In the case where Z<5X, numerous shortcarbon fibres are directed in the carbon fibre paper thicknessdirection, so there readily occurs breakage of the short carbon fibre ora short circuit occurs due to penetration of the polymer electrolytemembrane by the short carbon fibre. Here, the average length employed isthat based on the numerical average.

The meaning of the expression ‘the short carbon fibre is substantiallyoriented within a two dimensional plane’ is that the short carbon fibresare mostly lying flat. In this way, it is possible to prevent a shortcircuit with the opposite electrode due to the short carbon fibre orbreakage or the short carbon fibre.

As methods for ensuring that the short carbon fibre is substantiallyoriented within a two dimensional plane, there are the wet methodwhereby short carbon fibre is dispersed in a liquid medium andpapermaking carried out, and the dry method in which the short carbonfibre is dispersed in air and deposited as a pile. The wet method ispreferred for reliably ensuring that the short carbon fibre issubstantially oriented within a two dimensional plane and for increasingthe strength of the carbon fibre paper.

In order to enhance the strength and handling properties of the carbonfibre paper, it is preferred that there be present at least 30% of shortcarbon fibre of length at least 4 mm and preferably of length at least 6mm. More preferably, there will be at least 50% and still morepreferably at least 70%.

In order that the short carbon fibre be dispersed within a twodimensional plane, the upper limit of length will be no more than 30 mm,more preferably no more than 15 mm and still more preferably no morethan 8 mm. If the short carbon fibre is too long, then dispersionirregularities will readily arise. With regard to the dispersionirregularities, in the case for example where numerous short carbonfibres remain bunched together, the porosity in these bunched regionslowered, the thickness when pressure is applied is increased and so ahigher pressure is employed at the time of pressure application, andtherefore problems readily tend to arise such as damage to the carbonfibre paper, or localized thinning of the polymer electrolyte membraneor catalyst layer.

Again, it is preferred that the short carbon fibre be of a linear shape.If the length (L) of the short carbon fibre in its lengthwise directionis determined in a state free of external forces causing banding of saidshort carbon fibre, and the maximum divergence (Δ) between this length(L) and linearity is measured, then providing Δ/L is broadly no morethan 0.1, than the short carbon fibre may be regarded as linear. If theshort carbon fibre is linear, then it is possible to prevent morecompletely short-circuits occurring with the opposite electrode due tothe carbon fibre. Non-linear short carbon fibre readily tends to bedirected in three dimensions instead of being substantially randomlyoriented in a two-dimensional plane.

The short carbon fibre included in the carbon fibre paper should satisfythe relation given below between the diameter D (μm), the tensilestrength σ (MPa) and the tensile modulus E (MPa). Carbon fibre papercomprising such short carbon fibre is not readily damaged. In otherwords, the short carbon fibre does not readily break where the diameterof the short carbon fibre is low, the tensile strength high and thetensile modulus low, and the carbon fibre paper or a current collectoremploying the carbon fibre paper is not readily damaged at the time ofpressure application.

σ/(E×D)≧0.5×10⁻³

Here, the tensile strength and the tensile modulus of the carbon fibreare measured based on JIS R7601. In the case where the carbon fibre hasa flattened cross-section, the average value of the major and minor axesis taken as the diameter. In cases where short carbon fibres ofdifferent types are mixed together, weight-averaged values of D, σ and Erespectively are employed. Preferably, σ/(E×D)≧1.1×10⁻², and morepreferably σ/(E×D)≧2.4×10⁻³.

The tensile breaking elongation of the short carbon fibre is preferablyat least 0.7%, more preferably at least 1.2% and still more preferablyat least 1.8%. Short carbon fibre of tensile breaking elongation lessthan 0.7% is readily broken. The tensile breaking elongation is thevalue of the tensile strength (σ) divided by the tensile modulus (E).

Again, since breakage of the short carbon fibre arises under variouscircumstances, the tensile strength of the short carbon fibre ispreferably at least 500 MPa, more preferably at least 1,000 MPa andstill more preferably at least 2,000 MPa.

Moreover, it is preferred that the average length of the short carbonfibre be at least 4.5 mm and that it is at least seven times thethickness of the carbon fibre paper, and also that the diameter D (μm)of the short carbon fibre, the tensile strength σ (MPa) and the tensilemodulus E (MPa) satisfy the following relation.

σ/(E×D)≧1.1×10⁻³

With such carbon fibre paper, short circuiting of the polymerelectrolyte layer, breakage of the short carbon fibre and damage to thecarbon fibre paper do not readily occur, and the handling properties areexcellent, so it is ideal for use as the current collector of a polymerelectrolyte fuel cell.

The carbon fibre paper preferably shows a weight reduction of no morethan at when a uniform pressure of 2.9 MPa is applied in the thicknessdirection for two minutes and then the pressure removed. Thus, thecarbon fibre paper is not readily damaged when pressure is applied andit is possible to prevent a shortening of the fuel cell life due todamage to the current collector employing the carbon fibre paper.

The current collector employing the carbon fibre paper is subjected topressure in the thickness direction at the time of the coupling of thepolymer electrolyte membrane. the catalyst layers and the currentcollectors, and also when used as a fuel cell, so damage may occur.Again, when used as a cell, pressure is applied in the thicknessdirection in a state facing the grooved separators, so along with thefact that a high pressure is applied to those regions facing theprojecting parts of the grooved separators, those regions facing theboundaries between the projecting and depressed parts are also readilydamaged. If the current collector is damaged, shedding of broken shortcarbon fibres occurs, and a lowering in the current collector strengthand a raising of the electrical resistance in the planar direction arebrought about, and sometimes use as a cell is no longer possible. Inorder to prevent this from happening due to the is shedding of brokenshort carbon fibres and a lowering of the strength of the currentcollector, etc, it is necessary that the weight reduction is no morethan 3% when a uniform pressure of 2.9 MPa is applied in the thicknessdirection for two minutes and then the pressure removed. Preferably theweight reduction is no more than 2% and still more preferably no morethan 1%. With carbon fibre paper where the weight reduction is more than3%, there is a weakening following the removal of the pressure and ittends to be damaged by handling.

The measurement of the weight reduction is carried out as follows.Firstly, the carbon fibre paper is cut to a circular shape of diameter46 mm, and the weight measured. Next, the cut carbon fibre paper issandwiched between two glassy carbon plates of size larger than thecarbon fibre paper and having a smooth surface, then a pressure of 2.9MPa is applied over the area of the carbon fibre paper, and held for 2minutes. Subsequently, the pressure is removed and the carbon fibrepaper taken out. It is then dropped from a height of 30 mm with itsplanar direction perpendicularly disposed. After dropping in this wayten times, the weight is measured and the reduction in weightcalculated.

In order to prevent breakage of the short carbon fibre and keep thelevel of weight reduction to below 3%, it is preferred that the shortcarbon fibre employed be fibre produced by the cutting of continuouscarbon fibre. It is further preferred that tension be applied at thetime of the fibre heat treatment and it is still further preferred thatthe fibre be stretched at the time of the heat treatment. The carbonfibre used may a polyacrylonitrile (PAN) based carbon fibre, phenolicresin based carbon fibre or pitch based carbon fibre. Of these, the PANbased carbon fibre is preferred. When compared to pitch based carbonfibre. PAN based carbon fibre has a higher compression strength andtensile breaking elongation, and it breaks less readily. This is thoughtto be due to the difference in the crystallization of the carbon fromwhich the carbon fibre is composed. In order to obtain carbon fibrewhich does not readily break, the heat treatment temperature of thecarbon fibre is preferably no more than 2,500° C. and more preferably nomore than 2,000° C.

The diameter of the short carbon fibre which is contained in the carbonfibre paper is preferably no more than 20 μm. More preferably it is nomore than 12 μm and still more preferably no more than 8 μm. At thesurface of the carbon fibre paper contained in the current collector,gaps of diameter 5 to 10 times the diameter of the short carbon fibreare observed. At the time of the integral coupling with the catalystlayers, the faces of the polymer electrolyte membrane, the catalystlayers and the current collectors are made uneven by the short carbonfibre and by the gaps at the surface of the current collectors employingthe carbon fibre paper, and so the electrode reactions are facilitated.Hence, the diameter of the short carbon fibre should be low. If thediameter exceeds 20 μm, the radius of the gaps in the carbon fibre papersurface contained in the current collector is about the same as thethickness of the catalyst layer, and there is a lengthening of thedistance of electron flow between the catalyst particles in the catalystlayers and the short carbon fibres in the carbon fibre paper. Again, thefiner the short carbon fibre, the less readily it breaks when pressureis applied in the thickness direction. When short carbon fibres ofdifferent diameters are used, the diameter is determined by means of theweight average. On the other hand, if the diameter of the short carbonfibre is too fine, ingress by the catalyst layer into the currentcollector occurs less readily at the time of the integral coupling, so ashort carbon fibre diameter of at least 2 μm is preferred.

The volume resistivity of the carbon fibre contained in the carbon fibrepaper is preferably no more than 200 μΩ.m, more preferably no more than50 μΩ.m, and still more preferably no more than 15 μΩ.m. The measurementof the volume resistivity of the carbon fibre is carried out based onJIS R7601. Where the prescribed fibre length is not obtained, themeasurement is carried out at the obtained fibre length.

The resistance when a uniform pressure of 2.9 MPa is applied to thecarbon fibre paper is preferably 50 mΩ.cm² or below. More preferably itis 40 mΩ.cm² or below and still more preferably 20 mΩ.cm² or below.

In the measurement of the resistance, there are prepared two sheetscomprising copper foil of width 50 mm, length 200 mm and thickness 0.1mm superimposed on a glassy carbon plate with a smooth surface, of width50 mm. length 200 mm and thickness 0.15 mm. These are referred to as thetest electrodes. The two test electrodes are placed one on the other sothat they cross in the central region, with the glassy carbon platesfacing one another. The carbon fibre paper is cut to a circular shape ofdiameter 46 mm, interposed in the region where the glassy carbon platesoverlap and pressure applied to these glassy carbon plates so as to givea pressure of 2.9 MPa over the area of the carbon fibre paper.

Current terminals are provided at one end in the lengthwise direction ofeach of the plates and voltage terminals are provided at the other end.Using the current terminals, a current of 1 A is passed between the twotest electrodes. The voltage v (V) is measured between the voltageterminals and the resistance R (mΩ.cm²) calculated by means of thefollowing formula. Here, π is the circular constant pi.

R=V×2.3×2.3×π×1000

In order to lower the resistance, the carbon fibre heat treatmenttemperature is preferably at least 800° C. and more preferably at least1000° C.

The carbon fibre paper preferably has a thickness of 0.02 to 0.2 mm whena uniform pressure of 2.9 MPa is applied in the thickness direction.More preferably, it is from 0.04 to 0.16 mm and still more preferablyfrom 0.08 to 0.12 mm. If it is less than 0.02 mm, the current collectoremploying the carbon fibre paper is buried in the catalyst layer, andthe diffusion/permeation of the fuel or oxygen in the planar directionis lowered. If the thickness is greater than 0.2 mm, then the electricalresistance in the thickness direction is increased.

Carbon fibre paper of the aforesaid thickness when a uniform pressure of2.9 MPa is applied in the thickness direction preferably has a thicknessof 0.1 to 2.0 mm and more preferably 0.2 to 1.2 mm measured at apressure of 13 kPa. If it is more than 2 mm, then the carbon fibre paperhas high bulk, and short carbon fibre is directed in the thicknessdirection and the strength of the carbon fibre paper is weakened. Inorder to make the thickness less than 0.1 mm, it is necessary to carryout firm binding of the short carbon fibre with a considerable amount ofpolymer material.

The carbon fibre paper preferably has a density of 0.3 to 0.8 g/cm³ whena uniform pressure of 2.9 MPa is applied in the thickness direction.More preferably this is 0.35 to 0.7 g/cm³ and still more preferably 0.4to 0.6 g/cm³. The density of the carbon fibre paper when a uniformpressure of 2.9 MPa is applied in the thickness direction is determinedby calculation from the weight per unit area of the carbon fibre paperand the thickness of the carbon fibre paper when a uniform pressure of2.9 MPa is applied in the thickness direction.

It is sometimes necessary to raise the porosity in order to increase thediffusion/permeation properties of the current collector. If the densityis greater than 0.8 g/cm³ when a uniform pressure of 2.9 MPa is appliedin the thickness direction, the porosity is too low and thediffusion/permeation properties are inadequate. Again, if it is lessthan 0.3 g/cm³, then the resistance value in the thickness direction isincreased.

The carbon fibre paper preferably has a pressure loss of no more than 10mmAq when air is allowed to permeate at 14 cm/sec in the thicknessdirection in a state with no pressure applied. More preferably, this isno more than 3 mmAq, with no more than 1 mmAq still further preferred.

The weight per unit area of the carbon fibre paper is preferably from 10to 100 g/m2. More preferably it is from 20 to 80 g/m² and still morepreferably 25 to 60 g/m². At less than 10 g/m², not only is the strengthof the carbon fibre paper low but also there is thinning of the currentcollector when the polymer electrolyte membrane, the catalyst layers andthe current collectors are integrally coupled and at the time of cellassembly, and the current collector is buried in the catalyst layer sothat the diffusion/permeation effect in the planar direction isinadequate. If it exceeds 100 g/m², when assembled as a cell the currentcollector is thick and resistance increased.

The carbon fibre paper preferably includes fine carbon particles inorder to lower the resistance. The particle size of the fine carbonparticles is preferably no more than 3 μm, more preferably no more than0.5 μm and still more preferably no more than 0.1 μm. Carbon particlesof large particle size have little effect in lowering the resistance andthey reduce the diffusion properties. They also readily drop away fromthe current collector. As examples of the carbon particles, there arecarbon black, graphite powder and the like. Methods for incorporatingthe carbon particles include the method of binding the short carbonfibres by means of a polymer material containing fine carbon particlesand the method of affixing the fine carbon particles and the shortcarbon fibres by means of a polymer material. Binding the fine carbonparticles to the carbon fibre paper by means of a polymer material isalso preferred.

Now, the polymer electrolyte fuel cell unit includes at least a currentcollector employing the aforesaid carbon fibre paper and a catalystlayer, and short circuits through the polymer electrolyte membrane ordamage to the current collector do not readily occur. As the catalystlayer, there is used for example carbon powder an which the catalyst hasbeen supported, bound together with a fluoropolymer, and this is usuallyintegrally coupled to the current collector by coating or by pressbonding. Between the catalyst layer and the current collector there maybe provided a diffusion layer of carbon powder bound by a resin, but notproviding a diffusion layer and instead thickening the current collectorto a certain extent so that it jointly functions as a diffusion layer byproviding gas diffusion/permeation properties in the planar direction aswell as the thickness direction is preferred in terms of simplifying theproduction process.

The unit preferably has an integral structure. The application ofpressure is preferred either at the time of the integral coupling orsubsequent thereto, and simultaneous pressure application and heating ispreferred. Carrying out simultaneous pressure application and beating isparticularly effective in the case of integral coupling inclusive of thepolymer electrolyte membrane. Tho pressure applied is preferably 0.1 to20 MPa, with 0.5 to 10 MPa further preferred and 1.5 to 7 MPa stillfurther preferred. The heating temperature is preferably from 50 to 250°C. more preferably from 80 to 200° C. and still more preferably from 120to 180° C. By integral coupling, the contact resistance is lowered and,furthermore, as a result of the catalyst layers and polymer electrolytemembrane being roughened, there is the effect of lowering resistance,improving contact between the catalyst layer and the electrolytemembrane and raising the catalyst utilization, and by shortening thedistance from the current collector to the fine catalyst particleswithin the catalyst layer there is the effect that the electron transferdistance, and the hydrogen, oxygen and water supply/discharge routes,are shortened, so that the electrode reactions occur more readily. Withthis unit, there is the effect that short circuits through the polymerelectrolyte membrane and current collector damage are prevented whenused as a fuel cell, but in the case when the integral coupling isperformed by pressure application there is also the effect that shortcircuits through the polymer electrolyte membrane and current collectordamage due to this pressure applied at the time of integral coupling isprevented. Where heating is carried out at the same time as the pressureapplication at the time of the integral coupling, the polymerelectrolyte membrane is softened and the risk of a short circuit throughthe polymer electrolyte membrane is increased, so the effects ofpreventing short circuits through the polymer electrolyte membrane aremore markedly exhibited.

A polymer electrolyte fuel cell with current collectors or unitsemploying the carbon fibre paper of the present invention showsexcellent properties due to the aforesaid effects, and it is also aninexpensive polymer electrolyte fuel cell, so is ideal for drivingmoving bodies such as cars, trains, boats and the like.

In the production of a current collector employing carbon fibre paper,the method where there is included, prior to the formation of thecatalyst layer on the current collector, a stage in which pressure isapplied in the direction perpendicular to the carbon fibre paper face atthe same time as the heating of the carbon fibre paper causes the shortcarbon fibres to be oriented in a two dimensional fashion, and is aneffective method for preventing short circuits due to penetration of thepolymer electrolyte membrane and breakage of the short carbon fibres.

Moreover, in the production thereof, the method of applying pressure ina state with the carbon fibre paper soaked with liquid gives still moreoutstanding effects.

EXAMPLES Example 1

Carbon fibre paper was obtained by dispersing in water short PAN-basedcarbon fibre which had been cut to length 12 mm, then subjecting thedispersion to a paper-making process on a metal mesh, applying anemulsion comprising a mixture of polyvinyl acetate and PVA, which is apolymer material for binding the short carbon fibre, and drying. Thecarbon fibre paper, and the short carbon fibre and polymer materialused, are shown at the end of Table 1.

Examples 2 to 4

Carbon fibre paper was obtained in the same way as in Example 1,excepting that the weight per unit area, or the weight per unit area andthe polymer content, was/were altered. The carbon fibre paper, and theshort carbon fibre and polymer material used, are shown in Table 1.

Examples 5 to 8

Carbon fibre paper was obtained in the same way as in Example 2,excepting that the short carbon fibre was changed to a differentPAN-based short carbon fibre. The carbon fibre paper, and the shortcarbon fibre and polymer material used, are shown in Table 1.

Example 9

Carbon fibre paper was obtained in the same way as in Example 2,excepting that there was used an SBR emulsion as the polymer materialfor binding the short carbon fibre. The carbon fibre paper, and theshort carbon fibre and polymer material used, are shown in Table 1.

Examples 10 and 11

Carbon fibre paper was obtained in the same way as in Example 9,excepting that the weight per unit area, or the weight per unit area andthe polymer content, was/were altered. The carbon fibre paper, and theshort carbon fibre and polymer material used, are shown in Table 1.

Examples 12 to 15

Carbon fibre paper was obtained by dispersing, in water, PAN-based shortcarbon fibre and short fibre of a polymer material which binds the shortcarbon fibre, and then subjecting the dispersion to a paper-makingprocess on a metal mesh, followed by drying and binding by heating whileapplying slight pressure. The carbon fibre paper, and the short carbonfibre and polymer material used, are shown in Table 1.

Example 16

A phenolic resin based carbon fibre paper (produced by the Gun-EiChemical industry Co., “CP-22B”) was employed. The carbon fibre paper,and the short carbon fibre and polymer material used, are shown in Table1.

Example 17

Carbon fibre paper was obtained in the same way as in Example 2,excepting that there was used a mixture of PAN-based short carbon fibreand pitch-based short carbon fibre of weight ratio 1:1, the carbon fibrepaper, and the short carbon fibres and polymer material used, are shownin Table 1.

Examples 18 to 20

Carbon fibre paper was obtained in the same way as in Example 2,excepting that short fibre comprising a pitch-based carbon fibre wasused. The carbon fibre paper, and the short carbon fibre and polymermaterial used, are shown in Table 1.

Example 21

Carbon fibre paper was obtained in the same way as in Example 8excepting for the length of the short carbon fibre. The length of theshort carbon fibre and the number of dispersion irregularities are shownin Table 1. From Table 1, it is clear that the number of dispersionirregularities is reduced by shortening the length of the short carbonfibre.

Comparative Example 1

A pitch-based carbon fibre paper was employed (Kureha Carbon Fibre Paper“E-704”, produced by the Kureha Chemical Industry Co.). In this carbonfibre paper the short carbon fibre was bound by means of carbon, and thebulk density was 0.13 g/cm² (catalogue value).

Comparative Example 2

Carbon fibre paper was obtained by heat treating a rayon fibre paper for1 hour at 300° C. in air, and then heat treating at 2,200° C. in aninert atmosphere.

The properties of the carbon fibre papers in Examples 1 to 20 andComparative Examples 1 and 2 were measured. The results of themeasurements of the properties in Examples 1 to 20 are shown in Table 1.Comparative Examples 1 and 2 were damaged by 2.9 MPa pressureapplication. In the case of the carbon fibre paper of ComparativeExample 1, the carbon which binds the short carbon fibre was damaged bythe pressure. In the case of the carbon fibre paper of ComparativeExample 2, considerable breakage of the short carbon fibres occurred.

The following can be deduced from the results in Table 1.

The carbon fibre papers of the Examples show little weight reductionfollowing 2.9 MPa pressure application, and the resistance is low.

From a comparison of Examples 1 to 8 and Examples 18 to 20 it is clearthat the weight reduction following 2.9 MPa pressure application islower using short carbon fibre with a high value of σ/(E×D). Inparticular, there is a very considerable weight reduction of 7% inExample 19.

From a comparison of Examples 3 and 4 and Examples 10 and 11, it isclear that the resistance is lower the smaller the content of thepolymer material binding the short carbon fibre.

From a comparison of Examples 1, 2 and 4 and Examples 9 and 11, it isclear that the resistance is greater the lower the weight per unit areaof the carbon fibre paper and the lower the thickness when a pressure of2.9 MPa is applied in the thickness direction.

Again, when the weight reduction following pressure application at 2.9MPa exceeds 1%, then the feel begins to be softened. It is thought thatbreakage occurs even in short carbon fibre that does not drop away, andthere is an increased possibility of damage due to handling followingthe pressure application.

Example 22

The carbon fibre paper of Example 2, of dimensions 15 cm×15 cm, placedon a film, was impregnated with a liquid mixture of methanol and 0.056 gof phenolic resin, then the methanol evaporated by forced air drying andcarbon fibre paper obtained.

Example 23

The carbon fibre paper of Example 2, of dimensions 15 cm×15 cm, placedon a film, was impregnated with a liquid mixture of methanol, 0.005 g ofshort carbon fibre of length 3 mm and thickness 7 mm and 0.056 g orphenolic resin, then the methanol evaporated by forced air drying andcarbon fibre paper obtained.

Example 24

Carbon fibre paper was obtained in the same way as in Example 23excepting that there was used short carbon fibre of numerical averagelength 30 μm and thickness 7 μm.

Example 25

Carbon fibre paper was obtained in the same way as in Example 23excepting that there was used 0.001 g of short carbon fibre of length1.5 mm and thickness 7 μm.

Example 26

Carbon fibre paper was obtained in the same way as in Example 23excepting that there was used 0.003 g of short carbon fibre of averagelength 1.5 mm and thickness 7 μm.

Comparative Example 3

Carbon fibre paper was obtained in the same way as in Example 23excepting that there was used short carbon fibre of length 1.5 mm andthickness 7 μm.

Comparative Example 4

Carbon fibre paper was obtained in the same way as in Example 23excepting that there was used the carbon fibre paper of Example 21instead of the carbon fibre paper of Example 2, and there was used 0.07g of short carbon of length 1.5 mm and thickness 7 μm.

As a result of observation of the cross-sections of the carbon fibrepapers from Examples 22 to 25 and Comparative Examples 3 and 4 by eye,there were observed to be numerous short carbon fibres at an angle of45° or more from the plane of the carbon fibre paper in the case ofComparative Examples 3 and 4, while hardly any were to be seen in thecase of Examples 22 to 25.

With regard to Examples 22 to 25 and Comparative Examples 3 and 4,taking the thickness as X mm, the thickness at the time of 2.9 MPapressure application as Y mm, the average length of the carbon fibresexcluding those of length of (Y+0.2) mm or below as Z, and the length ofthe short carbon fibre as W mm, the proportion of short carbon fibressatisfying the relation W≧5X amongst the short fibres but excludingthose of length of (Y+0.1) mm or below was as shown in Table 1.

The following measurement was carried out as a test to simulate thepiercing of the polymer electrolyte membrane by short carbon fibreoriented in the thickness direction is of the current collector.

Two glassy carbon plates of length 200 mm and width 50 mm were placedcrossing one another, and carbon fibre paper was interposed in theregion of cross-over of the two carbon plates. At this time, the face ofone of these glassy carbon sheets in contact with the carbon fibre paperwas coated with 0.003 g/m² of silicone grease (Silicone Compound HVG,produced by the Toray Dow Corning Silicone Co.). Furthermore, to onelengthwise direction end of each of the two glassy carbon sheets therewas connected a terminal for current supply and at the other end therewas connected a terminal for voltage measurement. Pressure was appliedsuch that the glassy carbon sheets between which the carbon fibre paperhad been sandwiched exerted a pressure of 12 kPa on said carbon fibrepaper, and the voltage was measured along with a current flow of 30 mAbetween the two glassy carbon plates.

The voltage measurement results are shown in Table 1.

In the case of Comparative Examples 3 and 4, the voltage was lowered dueto the two effects of penetration of the silicone grease layer andpenetration of the carbon fibre paper by short carbon fibres of length1.5 mm disposed in the thickness direction of the carbon fibre paper. Inthe case of Example 23, the length of the short carbon fibre impregnatedalong with the phenol resin was long at 3 mm, so it is thought that itwas not possible for the fibre to form a large angle in terms of thecarbon fibre paper plane, and while there is penetration of the carbonfibre paper due to the short carbon fibres of length 3 mm, nopenetration of silicone grease layer occurs.

With the current collector for a polymer electrolyte fuel cell of thepresent invention, it is possible to prevent short-circuits through thepolymer electrolyte membrane due to the short carbon fibres and toprevent damage to the carbon fibre paper by applied pressure both duringelectrode production and during cell operation and, furthermore, it ispossible to achieve a comparatively low resistance in the thicknessdirection at the time of pressure application.

Such carbon fibre paper can be used as a current collector, as it is, orit can be employed as a current collector following post-treatment ofthe carbon fibre paper.

Industrial Utilization Potential

The current collector for a polymer electrolyte fuel cell of the presentinvention is inexpensive and, moreover, it is possible to prevent shortcircuits caused by short carbon fibre through the polymer electrolytemembrane. Furthermore, it is possible to prevent the short carbon fibrebreakage which occurs at the time of pressure application or damage dueto the failure of the binding by the polymer material. Consequently,there is obtained a polymer electrolyte fuel cell with little shortcircuiting due to the current collector, little increase in resistanceand little lowering of the diffusion/permeation properties.

What is claimed is:
 1. A carbon fiber paper for a polymer electrolytefuel cell, comprising carbon fibers bound with a polymer material,wherein, taking the carbon fiber paper as having a thickness of X mm,the thickness of the carbon fiber paper when 2.9 MPa pressure is appliedas Y mm and taking the carbon fibers as having a length of W mm, atleast 95% of the carbon fibers excluding those of a length of (Y+0.1) mmor less satisfy the relationship W≧5X.
 2. A carbon fiber paper for apolymer electrolyte fuel cell according to claim 1, wherein at least 98%of the carbon fibers excluding those of length (Y+0.1) mm or lesssatisfy the relationship W≧5X.
 3. A carbon fiber paper for a polymerelectrolyte fuel cell according to claim 1, wherein, taking the carbonexcluding those of length (Y+0.1) mm or less as having an average lengthof Z mm, the relationship Z≧5X is satisfied.
 4. A carbon fiber paper fora polymer electrolyte fuel cell according to claim 1, wherein the carbonfibers are substantially randomly oriented within a two dimensionalplane.
 5. A carbon fiber paper for a polymer electrolyte fuel cellaccording to claim 1, wherein the carbon fibers have a diameter D (μm),a tensile strength σ (MPa) and a tensile modulus E (MPa) that satisfythe following relationship: σ/(E×D)≧0.5×10⁻³.
 6. A carbon fiber paperfor a polymer electrolyte fuel cell according to claim 5, wherein thecarbon fibers have an average length of at least 4.5 mm and at leastseven times the thickness of the carbon fiber paper, and the followingrelationship is satisfied: σ/(E×D)≧1.1×10⁻³.
 7. A carbon fiber paper fora polymer electrolyte fuel cell according to claim 1, wherein the carbonfiber paper experiences a reduction in weight per unit area of no morethan 3% when a uniform pressure of 2.9 MPa is applied for 2 minutes in athickness direction of the carbon fiber paper and then the pressure isreleased.
 8. A carbon fiber paper for a polymer electrolyte fuel cellaccording to claim 1, wherein the carbon fibers are fibers of apolyacrylonitrile-based carbon fiber.
 9. A carbon fiber paper for apolymer electrolyte fuel cell according to claim 1, wherein the carbonfibers have a diameter of no more than 20 μm and a volume resistivity ina lengthwise direction of no more than 200 μΩ.m.
 10. A carbon fiberpaper for a polymer electrolyte fuel cell according to claim 1, having aresistance of no more than 50 μΩ.cm² when a uniform pressure of 2.9 MPais applied.
 11. A carbon fiber paper for a polymer electrolyte fuel cellaccording to claim 1, having a thickness of 0.02 to 2.0 mm and a densityof 0.3 to 0.8 g/cm³ when a uniform pressure of 2.9 MPa is applied.
 12. Acarbon fiber paper for a polymer electrolyte fuel cell according toclaim 1, having a weight per unit area of 10 to 100 g/m².
 13. A carbonfiber paper for a polymer electrolyte fuel cell according to claim 1,wherein the polymer material is 2 to 30 wt % of the carbon fiber paper.14. A carbon fiber paper for a polymer electrolyte fuel cell accordingto claim 1, further comprising carbon particles having a particle sizeof no more than 3 μm.
 15. A unit for a polymer electrolyte fuel cell,comprising a plurality of catalyst layers and a plurality of currentcollectors comprising a carbon fiber paper comprising carbon fibersbound with a polymer material, wherein, taking the carbon fiber paper ashaving a thickness of X mm, the thickness of the carbon fiber paper when2.9 MPa pressure is applied as Y mm and taking the carbon fibers ashaving a length of W mm, at least 95% of the carbon fibers excludingthose of a length of (Y+0.1) mm or less satisfy the relationship W≧5X.16. A polymer electrolyte fuel cell comprising a plurality of unitsaccording to claim
 15. 17. A moving body which is driven by a polymerelectrolyte fuel cell according to claim
 16. 18. The polymer electrolytefuel cell according to claim 16, wherein each of said plurality of unitsfurther comprises a polymer electrolyte membrane.
 19. The polymerelectrolyte fuel cell according to claim 18, wherein the polymerelectrolyte membrane is made of a fluoropolymer cation exchange resin.20. The polymer electrolyte fuel cell according to claim 19, wherein thepolymer electrolyte membrane has a thickness of about 0.05 to 0.15 mm.